Highly size- and shape-controlled synthesis of silver nanoparticles via a templated Tollens reaction.

A mild, facile one-step synthetic strategy for the preparation of size- and shape-controlled silver nanoparticles (AgNPs) is presented. The high degree of size- and shape-control of these AgNPs is achieved by the use of triazole sugar ligands scaffolded by a central resorcinol ether core. Both the triazoles and the resorcinol ether core mediate the nucleation, growth, and passivation phases of the preparation of AgNP in the presence of the Tollens reagent as the silver source. Kinetic and (1)H NMR titration data is presented describing the nature of the interactions between the Tollens reagent and these ligands.

The mechanism of formation of N-formylkynurenine by heme dioxygenases.

Heme dioxygenases catalyze the oxidation of L-tryptophan to N-formylkynurenine (NFK), the first and rate-limiting step in tryptophan catabolism. Although recent progress has been made on early stages in the mechanism, there is currently no experimental data on the mechanism of product (NFK) formation. In this work, we have used mass spectrometry to examine product formation in a number of dioxygenases. In addition to NFK formation (m/z = 237), the data identify a species (m/z = 221) that is consistent with insertion of a single atom of oxygen into the substrate during O(2)-driven turnover. The fragmentation pattern for this m/z = 221 species is consistent with a cyclic amino acetal structure; independent chemical synthesis of the 3a-hydroxypyrroloindole-2-carboxylic acid compound is in agreement with this assignment. Labeling experiments with (18)O(2) confirm the origin of the oxygen atom as arising from O(2)-dependent turnover. These data suggest that the dioxygenases use a ring-opening mechanism during NFK formation, rather than Criegee or dioxetane mechanisms as previously proposed.

Cu-catalyzed N-alkynylation of imidazoles, benzimidazoles, indazoles, and pyrazoles using PEG as solvent medium.

A facile and efficient Cu(I)-catalyzed cross-coupling method is reported for the preparation of N-alkynyl or N-bromoalkenyl heteroarenes from bromoalkynes. Generally superior yields and functional group tolerance were obtained with microwave (MW) irradiation using imidazole, benzimidazole, pyrazole, and indazole substrates and poly(ethylene glycol) 400 (PEG400) as an additive. We speculate that PEG400 acts as both a Cu(I)-stabilizing ligand as well as a phase transfer solvent.

Solid and solution state flexibility of sterically congested bis(imino)bipyridine complexes of zinc(II) and nickel(II).

Two new sterically demanding bis(imino)bipyridine ligands, 6,6'-{(2,6-i-Pr(2)C(6)H(3))N=R}(2)C(10)H(6)N(2) (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6'-(O=R)(2)C(10)H(6)N(2) (R = H, Me). Palladium(0)-mediated cross coupling of 2-(Bu(3)Sn)-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N with 2-Br-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N, followed by an acid-mediated deprotection, has been employed as an efficient route to the precursor 6,6'-bis(acetyl)-2,2'-bipyridine. Reaction of aldimino L1 with two equivalents of MX(2) [MX(2) = ZnCl(2), NiCl(2) or (DME)NiBr(2)] in n-BuOH at elevated temperature gives the five-coordinate mononuclear complexes [(L1)MX(2)] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M = Ni, X = Br 2b) as the sole products, in which one imine group is bound and the other uncoordinated (endo-exo). In the case of diamagnetic 1, VT (1)H NMR spectroscopy reveals a fast exchange process operating between the two possible forms of the endo-exo isomer which is likely to proceed via an exo-exo intermediate (DeltaH(double dagger)(interconversion) = 35.6 +/- 1.5 kJ mol(-1), DeltaG(double dagger)(298) = 47.4 +/- 3.3 kJ mol(-1)). In contrast, treatment of ketimino L2 with MCl(2) (MCl(2) = ZnCl(2) or NiCl(2)) affords bimetallic [(L2)Zn(2)Cl(4)] (4) and the six-coordinate monometallic species [(L2)NiCl(2)] (5), respectively; in 4, L2 adopts a bis(bidentate) bonding mode (endo-endo) while in 5 it acts as tetradentate ligand (endo-endo). Prolonged standing of 1 in chlorinated solvents results in partial hydrolysis and the formation of the 6-imino-6'-formyl-2,2'-bipyridine zinc complex, [(6-{(2,6-i-Pr(2)C(6)H(3))N=Me)-6'-(CH=)C(10)H(6)N(2))ZnCl(2)] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1,1 , 2a, 3, 4 and 5.

Interplay of structure and reactivity in a most unusual furan Diels-Alder reaction.

Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidly and in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the same conditions. These reactions have been explored using density functional theory (DFT) calculations. They reveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl(4) and by polar solvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding to the stepwise formation of the two new carbon-carbon bonds via transition states which have similar energies in all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predicted by our calculations. The calculated free energy barriers generally correlate well with measured reaction rates, supporting a reaction mechanism in which zwitterionic character is developed strongly. An in situ ring opening reaction of exo-cycloadduct ethyl exo-2-(N,N-diethylcarbamoyloxy)-3,3-difluoro-7-oxabicyclo[2.2.1]hept-5-enyl-2-endo-carboxylate, which results in the formation of cyclic carbonate ethyl 4,4-difluoro-5-hydroxy-2-oxo-5,7a-dihydro-4H-benzo[1,3]dioxole-3a-carboxylate by a Curtin-Hammett mechanism, has also been examined. Substantial steric opposition to Lewis acid binding prevents carbonate formation from 2-substituted furans.

Quantifying the reactivity of a remarkably long-lived difluorinated enol in acidic methanol via solution kinetics and electronic structure calculations.

A simple enol acetal underwent rapid cleavage in acidic solution to generate a difluorinated enol, which was sufficiently long-lived to be characterized by 2D NMR in a protic solvent at ambient temperature. Density functional theory calculations on a model reaction suggest that there are significant differences in protonation transition state timing between the fluorinated and nonfluorinated enols.

Highly-functionalised difluorinated cyclohexane polyols via the Diels-Alder reaction: regiochemical control via the phenylsulfonyl group.

A difluorinated dienophile underwent cycloaddition reactions with a range of furans to afford cycloadducts which could be processed regio- and stereoselectively via episulfonium ions, generated by the reaction between their alkenyl groups and phenylsulfenyl chloride. The oxabicyclic products were oxidised to the phenylsulfonyl level and ring opened via E1(C)B or reductive desulfonative pathways to afford, ultimately, difluorinated cyclohexene or cyclohexane polyols.

Towards novel difluorinated sugar mimetics; syntheses and conformational analyses of highly-functionalised difluorinated cyclooctenones.

Highly-functionalised difluorinated cyclooctenones were synthesised from trifluoroethanol using either metallated difluoroenol acetal or carbamate chemistry, followed by a [2,3]-Wittig rearrangement or aldol reaction. Efficient RCM reactions afforded the title compounds which showed rather restricted fluxional behaviour by VT (19)F NMR. Topological characterisation by molecular modelling and NOESY/ROESY experiments offered a number of challenges, but allowed the identification of two favoured boat-chair conformers which interconverted by pseudorotation with relatively large activation barriers.

A potentially divergent and rapid route to analogues of deoxycyclitols, pentopyranoses, 6-deoxyhexoses, and hexoses.

Direct precursors to analogues of pentopyranoses, 6-deoxyhexoses, and hexoses, in which a CF(2) center replaces the pyranose oxygen, have been synthesized rapidly from trifluoroethanol. A simple scaleable allylation reaction delivers ethers which undergo dehydrofluorination/metalation, followed by addition to either acrolein or cinnamaldehyde, to afford allylic alcohols. Fluorine-assisted [3,3]-rearrangement followed by reduction with sodium borohydride delivers diols, which undergo RCM smoothly to afford cyclohexene diols.

A stereodivergent asymmetric approach to difluorinated aldonic acids.

A (bromodifluoromethyl)alkyne has been deployed in a stereoselective route to difluorinated aldonic acid analogues, in which a Sharpless asymmetric dihydroxylation reaction and diastereoisomer separation set the stage for phenyl group oxidation.

Unexpectedly selective formation and reactions of epoxycyclooctenones under microwave-mediated conditions.

Topologically mobile difluorinated cyclooctenones undergo rapid, high-yielding, and completely stereoselective epoxidations with methyl(trifluoromethyl)dioxirane. The epoxides resist conventional hydrolysis but react smoothly in basic media under microwave irradiation to afford unique hemiacetals and hemiaminals in good yield. [reaction: see text]